Ferritic stainless steel composed of an Fe—Cr—Al-based alloy has excellent oxidation resistance at high temperatures, and therefore, is used in parts (e.g., catalyst carriers, a variety of sensors, and the like) for exhaust gas purifying devices for automobiles, motorcycles, marine bikes, motorboats, and the like. Furthermore, it is used in parts of frameworks for stoves, gas burners, and heating furnaces, and also is used in electric heating elements of heaters by taking advantage of its property of high electric resistivity. For example, Patent Literature 1 discloses a metal honeycomb using, instead of a conventional ceramic catalyst carrier, an Fe—Cr—Al-based stainless steel foil with a foil thickness of 20 to 100 μm having excellent oxidation resistance at high temperatures for the purpose of downsizing of catalyst carriers for exhaust gas purifying devices for automobiles and improvement in engine performance. Regarding this metal honeycomb, for example, flat stainless steel foils (flat foils) and corrugated stainless steel foils (corrugated foils) are alternately stacked to form a honeycomb structure, and a catalytic material is applied onto the surface of the stainless steel foils. The resulting metal honeycomb is used for an exhaust gas purifying device. FIG. 1 shows an example of a metal honeycomb. A metal honeycomb 4 is produced by winding a stacked structure including a flat foil 1 and a corrugated foil 2 into a roll shape, and fastening the outer periphery thereof with an external cylinder 3.
When the wall thickness of a catalyst carrier is reduced by using such a stainless steel foil, its thermal capacity is decreased. Consequently, the catalyst can be activated in a short period of time from engine starting, and exhaust back pressure can be decreased, thus being effective in downsizing of the catalyst carrier and improvement in engine performance.
On the other hand, automobile exhaust gas regulations are expected to be further tightened from the standpoint of environmental protection. In order to reduce nitroxides, carbon monoxide, and carbon hydrides emitted from gasoline vehicles and the like, techniques, such as one in which a catalyst carrier for an exhaust gas purifying device is placed at a position directly under the engine, close to the actual combustion environment, and a catalytic reaction is caused by high-temperature exhaust gas with the result that the amount of harmful substances in exhaust gas is reduced, have been developed. Furthermore, in order to improve fuel efficiency, engine combustion efficiency is enhanced, and the temperature of the exhaust gas itself is increased. Accordingly, catalyst carriers for automotive exhaust gas purifying devices are likely to be subjected to strong vibration in a harsher environment than before.
In order to cope with such a situation, stainless steel foils having various properties to catalyst carriers for automotive exhaust gas purifying devices have been proposed. For example, Patent Literature 2 discloses a heat-resistant stainless steel foil in which Mo and W are added into an Fe-20Cr-5Al alloy containing Zr, Hf, and a rare-earth element, and 1% to 15% of Ni is further added thereinto to precipitate NiAl, and thereby, proof stress (strength) at high temperatures is increased, resulting in improvement in durability. Furthermore, Patent Literature 3 discloses a stainless steel foil having excellent strength at high temperatures, which is a material for low thermal capacity and low exhaust gas pressure use, in which the foil thickness is less than 40 μm, and the Al content and the Cr content are changed in response to the foil thickness, and into which Nb, Mo, Ta, W, and the like are added. Patent Literature 4 discloses an alloy foil for a metal carrier in which proof stress at high temperatures and oxidation resistance are improved by adding Nb, Ta, Mo, and W into an Fe-20Cr-5Al alloy containing Y Mischmetal or mixed metals. Furthermore, Patent Literature 5 discloses a heat-resistant stainless steel foil in which proof stress at high temperatures are enhanced and durability is improved by adding a specific amount of Ta relative to C+N, Mo, W, and Nb into an Fe-20Cr-5Al alloy containing La, Ce, Pr, and Nd.